This application corresponds to French application 98 00936 of Jan. 23, 1998, the disclosure of which is incorporated herein by reference.
The present invention relates to the simultaneous deposition by winding on a support, of several rovings of pre-impregnated fibers.
One of the objects of the invention is to provide, by winding of rovings of fibers pre-impregnated with a resin and/or a suitable glue, an active skin of a panel of composite material with acoustic attenuation.
Generally speaking, such a panel is constituted of a sandwich comprising a central core of honeycomb structure covered on one side with a passive skin, impermeable to sound, and, on the other side, with an active skin, permeable to sound and hence provided with a suitable porosity.
Such an active skin can be made in several ways.
It can be constituted of one or several metallic sheets perforated regularly with calibrated holes.
It can also be constituted by a self-rigid metallic cloth, commonly called wire roving, reinforced for example with a cloth of carbon fibers of large roving to increase the structural strength of the exposed surface of the panel.
Controlling porosity, that is to say the amount of obduration, of the compound metallic cloth-carbon cloth is very difficult. If the proper porosity of each constituent is known and controlled, the final porosity of the assembly is often far from the desired theoretical porosity. The central reason for this phenomenon is the closing of certain of the interstices between the threads or filaments of said constituents by flow of the glue binding the constituents together as well as of the metallic cloth to the honeycomb structure.
In French patent application No. 97 09102, filed in the name of the assignee, there is proposed an acoustic attenuation panel whose active skin is made by winding rovings of pre-impregnated fibers, so as to obtain a skin constituted solely by fibers embedded in a resin matrix.
The winding of this active skin nevertheless requires a certain precision so as to give to the skin the desired permeability to sound between two successive windings with a suitable spacing.
However, this technique is limited to the production of a relatively thin skin, because the greater the thickness of the skin, not only is the porosity reduced but becomes uncertain, because the control of the precise positioning of the wound rovings is extremely difficult.
The use of this technique, shown for example by French patent application No. 97 10490, filed in the name of the assignee, for the production of a structural reinforcement on one of the surfaces of the metallic cloth instead of said carbon cloth with large rovings, does not permit solving satisfactorily the problem of precision of porosity, most particularly if it is desired to give to this structural reinforcement a substantial thickness greater than that of a single layer of wound rovings.
Another object of the invention is to provide a means to reduce the winding time of the rovings of pre-impregnated fibers on any support, which can be for example the winding mandrel itself, or an element connected to this mandrel, such as honeycomb structure, a metallic cloth or sheet perforated or not.
To these ends, the invention has for its object a process for depositing simultaneously by winding on a support, several rovings of pre-impregnated fibers, characterized in that it consists in superposing at least two rovings at the deposition place on the support, driven in rotation, said rovings being tensioned with different values in proportion to the range of superposition of the rovings, the roving nearest the support being more highly tensioned.
According to a first application, the winding is carried out so as to obtain a helicoidal winding with touching turns, in at least one layer.
According to a second application more specifically adapted to the production of an active skin permeable to sound, the winding is carried out so as to obtain a helicoidal winding with spires that do not touch, in at least one layer.
Spacing between two adjacent turns is constant along a generatrix of the rotatable support or variable locally along said generatrix.
The angle of inclination of the rovings to the axis of the rotatable support, generally constant from one end to the other of said support, is comprised between substantially 90xc2x0, the windings being of the circumferential type, and several degrees, the windings being of the longitudinal type.
The number of superposed rovings can be as desired and can exceed about ten and the physical characteristics or even the nature itself, of each roving can vary. Thus the section, the density of filaments, the width can vary from one roving to the other according to the objectives to be achieved.
The support on which are wound the rovings can be a metallic cloth, the complex thus obtained, after polymerization, being adapted to constitute an active skin of a panel of composite material for the attenuation of sound.
The support can also be a honeycomb structure, preliminary emplaced on a mandrel, the windings thus wound constituting the active skin of an acoustic attenuation panel.
According to a modification of the procedure used to produce an active skin, there are wound simultaneously several groups of superposed rovings, said groups being disposed side by side and separated by a separating filament or ribbon adapted to be withdrawn after polymerization of the resin of the fibers and/or of the glue.
This permits obtaining a very precise spacing between two adjacent turns of the helicoidal winding.
In the case of winding with touching turns, the winding support can be, as the case may be, either mandrel for direct winding followed by the emplacement of other elements such as a honeycomb structure, a cloth, a metallic sheet, etc. . . . , or one or several elements first emplaced on a mandrel, such as a skin or a honeycomb structure, or again a structure serving as a rotatable mandrel, such as a metallic envelope for the production of receptacles for storage of fluid under high pressure.
Pre-impregnation with the help of the resin and/or suitable glue, of the fibers of the rovings, is carried out upon production of the rovings or else in situ by passing the rovings through a suitable bath just before depositing them on the rotatable support.
The invention also has for its object a device for practicing the process, characterized in that the specific means to emplace and superposition the rovings, giving them the suitable tension, guiding them laterally if desired just before their deposition on the rotatable support, or again of the means to apply and position the groups of superposed rovings with a separator in the modification mentioned above.
The invention finally has for its object any structure or a part of a structure including a skin or cladding constituted of rovings deposited according to the process.